Generating Electricity P
Visão Geral
Switch on a lamp and the energy arriving at it left a power station a fraction of a second earlier, produced by nothing more exotic than a loop of copper wire moving past a magnet. Coal, gas, uranium, falling water, wind: they argue about everything except the last step, and the last step is always the same. Spin a coil between the poles of a magnet and a potential difference appears across its ends. Almost every joule of electrical energy used on the planet today was made that way.
This lesson takes that one effect apart. You will find out what has to happen before a pd appears at all, what makes it large rather than tiny, and why the coil fights back against whoever is turning it, which turns out to be the reason the whole arrangement cannot cheat. Then you will read the two output graphs that separate an alternator from a dynamo, and see why a dynamo's so-called direct current is nothing like the steady output of a cell.
Objetivos
- A potential difference (pd) is induced across the ends of a conductor when: the conductor moves relative to a magnetic field; the conductor is in a changing magnetic field. This is called the generator effect. Students should be able to suggest the factors that affect the size of the induced pd.
- A potential difference is induced across the ends of a coil of wire when: a permanent magnet is moved into or out of the coil; the coil is moved relative to the magnet.
- If the conductor is part of a complete circuit, a current flows in the wire. The magnetic field produced by the induced current opposes the field of the permanent magnet.
- If the direction of motion, or the polarity of the magnet, is reversed, the polarity of the induced potential difference and direction of flow of any induced current are reversed.
- The size of the induced potential difference increases when: the speed of the movement increases; the strength of the magnetic field increases; the number of turns on the coil increases; the area of the coil increases. Students should be able to explain how an alternator generates ac and a dynamo generates dc, including graphs of potential difference generated in the coil against time. However, detailed knowledge of slip rings and split rings are not required.
- Power stations use turbines to turn wire coils between magnets to generate electricity.
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- A bar magnet is held completely still inside a coil of wire connected to a centre-zero meter. What does the meter read? A. Zero B. A steady large reading C. A steady small reading D. A reading that slowly increases Answer: A
- Which change would NOT increase the size of the potential difference induced in a coil by a magnet? A. Moving the magnet into the coil more quickly B. Using a stronger magnet C. Winding more turns onto the coil D. Using a coil of smaller area Answer: D
- A magnet is pushed into a coil north pole first and a current is induced. The magnet is then pushed into the coil south pole first at the same speed. What happens to the induced current? A. It is unchanged B. It reverses direction and keeps the same size C. It keeps its direction and becomes smaller D. It falls to zero Answer: B
- Which description matches the graph of induced potential difference against time for a dynamo? A. A horizontal straight line above the zero line B. A smooth wave with equal parts above and below the zero line C. A series of identical humps that touch the zero line but never go below it D. A straight line rising steadily from the origin Answer: C
- In a power station, what is used to turn the coils of wire between the magnets? A. A transformer B. A turbine C. A battery D. An electromagnet Answer: B
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